Vibration analysis has been used for years to provide a determination of the proper functioning of different types of machinery, including rotating machinery and rocket engines. A determination of a malfunction, if detected at a relatively early stage in its development, will allow changes in operating mode or a sequenced shut down of the machinery prior to a total failure. Such preventative measures result in less extensive and/or less expensive repairs, and can also prevent a sometimes catastrophic failure of equipment.
Standard vibration analyzers are generally rather complex and difficult to transport, on a routine basis, to machinery for vibration analysis purposes. Due to this logistics problem and the necessary complexity of such systems, a high cost is often associated with standard vibration analyzers. If vibration analyzer systems are permanently positioned adjacent machinery to be monitored, they may be used to provide control signals in response to vibration detected. However, if the control circuitry is located some distance from the vibration analysis system, there may be errors produced due to ground shifts/ground loops which typically occur in the harsh and electrically noisy operating environments in which rotating machinery operate. Ground loops/ground shifts, as used herein, are voltage differences between various circuit grounds. These voltage differences between grounds are typically variable, unknown, and tend to cause false signals. If the detector circuitry for the vibration analysis equipment and accelerometer transducer are located some distance apart and/or powered by different power supplies, those components may also be subject to ground loop errors.
Less complex and costly hand held vibration analyzers typically do not provide adjustable trip point signals to actuate alarms or shutdown equipment in response to sensed vibrations. Since consistent long term measurement of vibration may be sensitive to precise accelerometer sensor location and orientation, it may be difficult to monitor long term machinery condition using hand held equipment that does not provide for permanent fixture of the accelerometer sensor. The circuitry of hand held vibration monitoring equipment is generally not suitable for permanent mounting to machinery to be observed.
A panel mounted accelerometer display is disclosed in U.S. Pat. No. 4,622,548 to J. R. Andres and D. D. Wilson. The G-force acceleration measuring instrument is designed for use in aircraft, spacecraft, submarine, or other vehicles, and employs electronic sensing and display transducers for increased reliability with relatively small size and small mass. The instrument includes a bar graph display, preferably incorporating liquid crystal transducer elements. It uses a solid state or limited motion acceleration transducer and provides graphic and numeric indication of the present G-force value together with number indication of historic maximum and minimum G-force values. The instrument also includes gain adjustment arrangements for accommodating different instrument panel mounting and different types of vehicles with different G-force capability, and has a signal output tap for recording or slave indicating or telemetry or feed-back use. Since this device is designed to be panel mounted, there are no provisions for retro-fitting it to an existing commercial accelerometer to form an integral unit to save costs of construction. No means are disclosed for vibration isolation of the electronics. For this reason it is probably not suitable for direct "on machine" mounting. It is also rather bulky for such installation. While the unit is of small size relative to other panel mount devices, it is relatively large and bulky for machine mounting. The electronics may be too complex for "on machine" mounting and include analog to digital components as well as sample and hold circuits. There is no means for providing a digital trip signal or for varying the conditions which would produce such a trip signal. No velocity signal output is available. While the scales may be changed, the device is limited to linear scales and cannot readily be changed for monitoring with logarithmic scales.
Purely mechanical accelerometers are available for monitoring purposes but do not have a means for providing electrical error or trip signals. Purely mechanical accelerometers may be difficult to read unless external lighting is provided. Mechanical accelerometers have a fixed scale so that different accelerometers must be used for different machines depending on the expected range of vibration to be measured. Typically, there is no convenient means to change the range of measurement of a mechanical accelerometer.
U.S. Pat. No. 4,198,864 to D. S. Breed discloses a mechanical apparatus for sensing and recording changes in the velocity of a vehicle involved in a crash. It is comprised of a sensing mass biased to an initial position. The biasing force is only overcome by an acceleration having a certain magnitude or greater. The mass will move a distance proportional to the force and will remain in position if the magnitude and duration of force are sufficient to overcome preset values. This device is suitable for measuring forces in a single catastrophic occurrence but is not suitable for on-going monitoring of equipment.
U.S. Pat. No. 4,470,302 to N. E. Cart discloses a mechanical shipping accelerometer having a transparent tube within which an inertial mass moves indicators relative to a fixed scale affixed to the transparent tube to provide an indication of maximum shock incurred in either direction of its longitudinal axis. Springs on either side of the inertial mass maintain the inertial mass in its initial position. As with other mechanical accelerometers, the absence of an electronic trip signal, the ability to change scales, and a lack of an illuminated scale limits its usefulness.
Consequently, a need exists for improvements in accelerometer electronic displays and control functions which are more suitable for operation directly on machines and which are not so expensive and complex. Those skilled in the art have long sought and will appreciate the novel features of the present invention which solves these problems.